As to a conventional technology, a surface acoustic wave device (hereinafter, referred to as a “SAW device”) is described hereinafter as an example.
Recently, a large number of small and light SAW devices have been used in electronic equipment such as various types of mobile communication terminal devices. In particular, in a radio circuit part of a mobile phone system within a band of 800 MHz to 2 GHz, a surface acoustic wave filter formed by using a lithium tantalate (hereinafter, referred to as “LT”) substrate has been widely used. However, an LT substrate has a large thermal expansion coefficient of a substrate in the direction in which a surface acoustic wave propagates. Furthermore, the elastic constant itself varies according to temperatures. Therefore, there is a problem in terms of the temperature characteristics that the frequency characteristic of a filter is also greatly shifted according to the change in temperatures.
In such circumstances, for example, Japanese Patent Unexamined Publication No. 2004-254291 (patent document 1) discloses a method of obtaining a SAW device in which the temperature characteristics is improved. The SAW device described in patent document 1 includes a piezoelectric substrate, an electrode film and an insulating film. The electrode film is formed on the piezoelectric substrate and constitutes at least one IDT. The insulating film is formed by sputtering on the piezoelectric substrate so as to cover the electrode film. In addition, the insulating film has unevenness on the upper surface thereof. Furthermore, when the film thickness of the electrode film is in the range from 1-3% of the wavelength of the surface wave to be excited, excellent electrical characteristics can be obtained.    [Patent Document 1] Japanese Patent Unexamined Publication No. 2004-254291